The present invention relates to an improved method for impregnating and coating wire rope with a curable polymeric material, and the wire rope produced by the improved method. More particularly, the present invention relates to a method for continuously impregnating and encapsulating wire rope with a polymer formed from at least two reactive components which when mixed together and heated react chemically to form a curable polymeric material.
Various methods are known for impregnating wire rope with plastic material. Such impregnated wire rope reduces contact stress between the strands of the rope and improves the load bearing capacity of the strands. Encapsulating or coating the outer surface of the wire rope in a plastic jacket also serves to protect the strands from corrosive elements such as weather and dirt, as well as sealing lubricant within the plastic jacket.
Presently known methods employed for the plastic impregnation of wire rope utilize dies which extrude plastic material around the wire rope or around the strands which are then formed into wire rope. These methods are satisfactory, but entail certain disadvantages. For example, present extrusion methods may cover the exterior surface of the wire rope with plastic, but the high viscosity of the plastic may not permit its sufficient penetration into the interstitial voids or spaces between the strands. A lack of penetration of plastic material into the voids permits abrasive contact between bare strands. High viscosity plastics are also difficult to extrude, and sometimes clog the extrusion apparatus.
Drastically decreasing the viscosity of the plastic material to the extent necessary for total impregnation of plastic material in and around the strands increases the curing time of the plastic material. In addition, the use of very low viscosity plastic material increases the expense associated with the extrusion apparatus and process by requiring the apparatus to be tightly sealed to inhibit leaks of plastic material. In addition, a uniform coating of plastic is more difficult to obtain when using ultra-low viscosity plastics.
In accordance with known methods, it is most preferable to maintain the plastic material in an intermediate viscous state just prior to and during the extrusion process to ensure a uniform coating and prevent clogging of the extrusion apparatus. Maintaining the plastic material in the proper viscous state requires a large energy input and therefore constitutes a significant expense in the manufacture of impregnated wire rope using conventional methods.
In U.S. Pat. No. 4,098,861 to Bassini, a method is disclosed for coating a single wire using a low viscosity multi-component polymer system in which two or more reactive, low viscosity ingredients are blended to form a rapidly reacting mixture which cures to form a solid polymer around the wire. The use of a multi-component polymer system is sometimes referred to as reactive injection molding (RIM) technology. Such techniques have the advantage that the components are not mixed until ready for use and thus it is less expensive to maintain the components and/or the mixture in the proper viscous state.
The Bassini process or RIM technology may be satisfactory for coating a single wire having a smooth outer surface without internal spaces or voids. However, it is unsuitable for use with a wire rope or strand having a convoluted outer surface and interstitial voids. In particular, the process parameters of Bassini (e.g., temperatures of 175.degree.-400.degree. F.; pressures of 200-3500 psi; curing period of approximately 1 second; and coating speeds of approximately 3000 feet per minute) render it incapable of uniformly impregnating and encapsulating a wire rope or strand in a continuous economic process. In the Bassini process, the relatively quick coating speed and curing period do not allow sufficient time for the material to impregnate or flow into the voids of the wire rope. In addition, the high pressures of Bassini may result in the material flowing or leaking from the coating device.